Microsoft update servers left all Azure RHEL instances hackable

Patch proffered, pen-tester paid

Software engineer Ian Duffy found the flaws while building a secure RHEL image for Microsoft Azure. During that process he noticed an installation script Azure uses in its preconfigured RPM Package Manager contains build host information that allows attackers to find all four Red Hat Update Appliances which expose REST APIs over HTTPS.

From there Duffy found a package labelled PrepareRHUI (Red Hat Update Infrastructure) that runs on all Azure RHEL boxes, and contains the rhui-monitor.cloud build host.

Duffy accessed that host and found it had broken username and password authentication. This allowed him to access a backend log collector application which returned logs and configuration files along with a SSL certificate that granted full administrative access to the four Red Hat Update Appliances.

Duffy says all Azure RHEL images are configured without GPG validation checks meaning all would accept malicious package updates on their next run of yum updates.

"In theory, if exploited one could have gained root access to all virtual machines consuming the repositories by releasing an updated version of a common package and waiting for virtual machines to execute yum update," Duffy says.

"[Compromising updates] would just be a case of bumping the version number and releasing a package under the same name."

Microsoft shuttered access to rhui-monitor.cloud and rotated secrets to close the hole.

Duffy found another vulnerability within the mandatory Microsoft Azure Linux Agent (WaLinuxAgent) which exposed API keys for debugging purposes.

The flawed Agent made it possible for Duffy to gain administrator API keys and download virtual hard disks for any RHEL using the same storage account.

Duffy says he was paid less than US$3500 for the vulnerability disclosures under Microsoft's bug bounty but did not name a precise figure. ®